Add seed.bchd.cash to seeder lists

Summary: They run mainnet seeder only, no testnet.

Test Plan:
make check
Type nslookup seed.bchd.cash and check that it returns a list of peers.

Reviewers: Fabien, deadalnix, #bitcoin_abc, jasonbcox

Reviewed By: deadalnix, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4073